Thyratron welding control



y 1939- L. G. LEVOY. JR., El AL 2,166,305

THYRATRON I WELDING CONTROL Filed June 2, 1957 2 Sheeis-Sheet 1 FI'QJ.

Inventors: Louis G. Levog Jn, George W. Garman,

cf 41:4 by Their' Aizorneg.

| G. LEVOY, JR.. ET AL 2,166,305

THYRATRON WELDING CONTROL July 18, 1939.

Filed June 2, 1957 2 Sheets-Sheet 2- N V V U UV \ um m Inverfitor-s:

LouisGLevoy Jn, George W. G arman Attorney.

Patented July 18, 1939 UNITED STATES PATENT OFFICE THYRATRON WELDINGCONTROL Application June 2, 1937, Serial No. 146,020

8 Claims.

Our invention relates to electric translating apparatus and moreparticularly to control circuits for electric valve translatingapparatus.

Heretofore there have been devised numerous ll circuits for controllingthe operation of electric valve translating apparatus to effect periodicor intermittent energization of a load circuit from an alternatingcurrent supply circuit. Many of these arrangements have employedelectric valve i circuits for generating periodic electrical quantities,such as voltages or currents, to control the periodicity of theenergization of the load circuit and these arrangements have also beenemployed to control the amount of energy transmitted to thz load circuitduring each period of energizatiO'l and for controlling the ratio of theintervals of conduction to the intervals of nonconduction. However, inthese prior art arrangements there has been evidenced considerabledifficulty in et- 29 footing the desired flexibility ofcontrol of theperiodicity of the energizations without sacrificing the highlydesirable precision of control of the ratio of the intervals ofconduction to the intervals of nonconduction. For example, in elec- 25tric welding circuits where it is desirable to provide periodicenergizations of the welding circuit, it is also desirable to providecontrol circuits in which the amount of energy transmitted to the loadcircuit and the ratio of the intervals of conduction to the intervals ofnonconduction are accurately and precisely determinable andcontrollable.

It is an object of our invention to provide a new and improved electricvalve translating circuit. 35 It is another object of our invention toprovide a new and improved control system for electric valve translatingapparatus.

It is a further object of our invention to provide a-new and improvedelectric valve circuit for 40 controlling an electric valve translatingapparatus to effect periodic or intermittent energization of anassociated load circuit in which the ratio of the intervals ofconduction to the intervals of nonconduction is precisely and accuratelycontrollable.

In accordance with the illustrated embodiment of our invention, weprovide an electric valve translating circuit for transmitting energyfrom an alternating current supply circuit to a load 50 circuit, such asa welding circuit. A control sysan accurately determinable interval oftime in accordance with a single circuit controlling opera tion. Moreparticularly, the control system includes an electric valve circuit forgenerating an electrical impulse, or a train of electrical impulses, todetermine the periodicity of the energization of the load circuit. Thegeneration of the electrical impulses may be controlled so thatconsecutive impulses occur during half cycles of voltage of oppositepolarity of the alternating cur l0 rent supply circuit or during halfcycles of voltage of a predetermined polarity of the supply circuit. Theelectric valve control system also includes an independent secondcircuit for generating a voltage which controls the period ofenergization of the load circuit and the amount of energy transmittedthereto, or when the system is operating to effect intermittent orperiodic energization of the load circuit, the second circuit controlsthe amount of energy transmitted to the load circuit and controls theratio of the intervals of conduction to the intervals of nonconductionof the main or power electric valves.

For a better understanding of our invention, reference may be had to thefollowing description taken in connection with the accompanying drawingsand its scope will be pointed out in the appended claims. Fig. l of theaccompanying drawings diagrammatically shows an embodiment of ourinvention as applied to an electric valve translating system forenergizing a welding circuit; and Fig. 2 represents certain operatingcharacteristics thereof.

In Fig. 1 of the accompanying drawings our invention is showndiagrammatically as applied to an electric valve translating system fortransmitting energy from an alternating current supply circuit I to aload circuit, such as a welding circuit 2, through a transformer 3having a primary winding 4 and a secondary winding 5. A suitable circuitcontrolling means, such as a switch 6, may be interposed between thesupply circuit I and transformer 3. A pair of oppositely connectedelectric valves 1 and 8 are connected in series relation with theprimary winding 4 of transformer 3 and serve to control the voltageimpressed on the winding 4 and to thereby control the currenttransmitted to the welding circuit 2. The electric valves 1 and 8 arepreferably of the type employing an ionizable medium, such as a gas or avapor, and in the particular arrangement illustrated in the drawingsthese valves are shown as being of the type employing an anode 9, amercury pool cathode I0 and a control member ll of the immersion-ignitertype.

To control the energization of the control members ii of electric valvesI and 6 to control the periodicity of the energization of load circuit 2from supply circuit I, we employ a circuit l2 which generates periodicelectrical impulses or generates a predetermined number of periodicelectrical impulses in response to a single circuit controllingoperation. The circuit I 2 may be of the type disclosed and claimed in acopending application of Harold W. Lord, Serial No. 146,019 filedconcurrently herewith and assigned to the assignee of the presentapplication. The circuit l2 includes a source of directcurrent l3. Thesource of direct current l3 may be supplied by any suitable arrangementsuch as a bi-phase rectifier l4 including a transformer l5, which isconnected to the alternating current circuit I, and a pair ofunidirectional conducting devices I6 and I1. The circuit I2 alsoincludes a voltage divider including serially-connected resistances l8,l9 and 26 which are connected across the source of direct current l3,and includes a second voltage divider including resistances 2|, 22 and23 which are connected in series relation across the direct currentsource I3. A pair of parallel electric paths are connected to beenergized from the direct current source I3; one of these paths includeselectric valve 24 and a serially-connected impedance element 25 and theother parallel path includes electric valve 26 and a serially-connectedimpedance element 21. The electric valves 24 and 26 are preferably ofthe type employing an ionizable medium, such as a gas or a vapor, andeach includes an anode 26, a cathode 29 and a control member 36. Acapacitance 3| is connected across the parallel paths and is arranged tobe charged alternately in opposite directions through electric valves 24and 26 from the direct current source l3. An inductive device, such as atransformer 32 having a primary winding 33 connected in theanode-cathode circuits of electric valves 24 and 26 is employed tosupply alternating voltages in response to the electrical impulsesgenerated in the circuit l2 due to the charging of the capacitance 3|.The transformer 32 is also provided with a secondary winding 34 having aneutral connection 35.

Suitable impedance means including. capacitances 36 and 31, resistances36, 39 and 46 and circuit controlling means, such as a switch 4|, may beconnected in the circuit l2 tocontrol the period of the electricalimpulses generated by circuit l2. A'switch 42 may be connected in thecircuit to provide additional flexibility in the control of theperiodicity of the electrical impulses generated by this circuit.

As a means for controlling the circuit l2 -to generate electricalimpulses of which consecutive impulses occur during half cycles ofvoltage of opposite polarity ofthe supply circuit I or during halfcycles of voltage of a predetermined polarity of the circuit I, weemploy an excitation circuit including a suitable circuit controllingmeans, such as a reversing switch 43, which impresses on control members36 of electric valves 24 and 26 alternating voltages displacedelectrical degrees or in-phase alternating voltages. The switch 43 ispreferably of the snap acting type biased to the left-hand position andto the right-hand position. An impedance 44 of relatively large value isconnected across the switch 43 to prevent the grids 36 of electricvalves 24 and 26 from floating during switching operations. Theexcitation circuit also includes a 'trol on circuit transformer 45having a primary winding 46 and secondary windings 41 and 46 whichsupply the alternating voltages for energizing control members 36 ofelectric valves 24 and 26. Primary winding 46 is connected toalternating current circuit I. The voltage supplied by the secondarywinding 41 is impressed across cathode 29 and control member 36 orelectric valve 24 through a circuit including a capacitance 45, and thevoltage supplied by secondary winding 48 is impressed across cathode 29and control member 36 of electric valve 26 through a circuit includingcapacitance 56. A pair of serially connected resistances 5| and 52 areemployed to impress on control members 36 of electric valves 24 and 26 aunidirectional negative biasing potential to be discussed hereinafter.

As anagency for selectively energizing the control members 36 ofelectric valves 24 and 26 of control circuit l2 to generate intermittentor periodic electrical impulses or to cause circuit l2 to generate onlya predetermined number of electrical impulses, we employ a suitablecircuit controlling means. such as a switch 53. When the switch 53 is inthe left-hand position the circuit is connected to generate a train ofperiodic electrical impulses, and when the switch is in the right-handposition the excitation circuit for electric valves 24 and 26 isconnected to a circuit 54 which modifies the negative unidire c tionalbiasing potential impressed on control members 36 to permit these valvesto generate only a predetermined number of electrical impulses, or onlyone electrical impulse. The cir cult 54 may be of the type disclosed andclaimed in the above identified copending application 01, Harold W.Lord. .The circuit 54 includes a serially connected resistance 55, acapacitance 56 and a glow discharge valve 51 which are connected acrossthe direct current source 13. impedance element, such as a resistance 56of relatively large value, is connected across the glow dischargevalve51 and serves as a means for impressing across the terminals thereof arelatively large transient voltage to render the valve conductive when aparallel circuit including a resistance 56 and a circuit controllingdevice, such as a switch 66, is opened. The 011'! I cuit including theresistance 59 and switch 66 is connected across the seriallyconnectedcapacitance 56 and the glow discharge valve 51. A voltagedivider, including a serially connected resistance 6| and a capacitance62 is responsive to the voltage of the valve 51, and is connected acrosselectric valve 51 to supply a negative unidirectional biasing potentialwhich is a predetermined component of the voltage appearing across the.electric valve 51. voltage is transmitted to the switch 53 for electricvalves 24 and 26 through a conductor 63. An impedance 64 of relativelylarge value is connected in series relation with the conductor 63 andthe excitation circuit for electric valves 24 and 26 and serves torender ineifective circuit 54 when the switch 53 is in the left-handposition. When the switch 53 is in the right-hand position, theimpedance 64 is effectively short' circuited to permit circuit 54 toexert its conl2. As a means for adjusting the resultant negativeunidirectional biasing potential impressed on control members 36 ofelectric valves 24 and 26, we employ a voltage iividerincluding aresistance 65 having anad- \lstable tap 66 which is energized inaccordance This component of.

with the voltage appearing across a predetermined portion of resistanceIII.

A circuit 81 is employed to generate a periodic voltage or an impulse ofvoltage which controls the amount of energy transmitted to the loadcircuit 2 during each period of energization thereof, or which may beemployed to control the ratio of the intervals of conduction to theintervals of nonconduction of the electric valves I and 8 duringperiodic energization of the load circuit 2, or which may be employed tocontrol the duration of the period of energization of the load circuit 2when the system operates to effect only one internal of energizationthereof. The circuit 81 includes a capacitance 68 which is connected tobe charged from the direct current source I3 through a seriallyconnected resistance 88 and an adjustable impedance element such as anadjustable resistance I0. To discharge the capacitance 88 in response tothe electrical impulses generated by circuit I2, we provide a seriallyconnected inductance II and an electric valve 12 which are connectedacross capacitance 68. The electric valve 12 includes an anode I3, acathode I4 and a control member 15 and is preferably of the typeemploying an ionizable medium such as a gas or a vapor. A capacitance I6may be connected across the control member I5 and cathode I4 to absorbextraneous transient voltages and a current limiting resistance I6 maybe connected in series relation with the control member I5. A parallelconnected capacitance I1 and a resistance I8 are connected in seriesrelation with the parallel circuits of circuit I2 to establish incircuit I2 an additional bias voltage. The circuit 61 is connected tothe left-hand terminals of capacitance TI and resistance I8.

We employ an excitation circuit for energizing control member I5 ofelectric valve I2 including a rectifier I8, energized in accordance withthe voltage variations of secondary winding 34 of transformer 32, andincluding unidirectional conducting devices and 8I. The rectifiedimpulses of alternating current transmitted through resistance 82impress on control member I5 of electric valve I2 impulses of voltagewhich render electric valve I2 conductive. A parallel connectedcapacitance 83 and resistance 84 are connected in series relation withresistance 82 to impress on control member "I5 of electric valve I2peaked or sharply defined impulses of voltage.

To control the magnitude of the periodic voltages generated by thecircuit 61 so that the impulses of this voltage are symmetrical and ofcorresponding magnitude, I provide a unidirec tional conducting device85 which is connected between the common juncture of capacitance 68 andcathode I4 of electric valve I2 and a point 88 on resistance 22. Thefeatures including the excitation circuit for electric valve I2including the rectifier circuit I5 and the arrangement for controllingthe magnitude of the periodic voltage generated by circuit 61 includingelectric valve 85 are disclosed and claimed in a copending applicationof Louis G. Levoy, Serial No. 146,021 filed concurrently herewith andassigned to the assignee of the present application.

The voltage generated by circuit 81 is employed to control theconductivities of electric valves I and 8. A circuit 81 including anelectric valve 88 controls the energization of immersion-igniter controlmembers II of electric valves I and 8. Electric valve 88 is providedwith a control member 89 and is preferably of the type employing anionizable medium such as a gas or a vapor. The voltage generated by thecircuit 61 is impressed on control member 89 of electric valve 88through a circuit including a conductor 90 and a serially connectedresistance 9|. A cathode 92 of electric valve 88 is connected to thecommon juncture of resistances 2I and 22 through a conductor 83. Thecircuit 81 is disclosed and broadly claimed in a copending applicationof Harold W. Lord, Serial No. 138,809, filed April 24, 1937 and assignedto the assignee of the present application. The circuit 81 controlselectric valves I06 to energize control members II of electric valves 1and 8, and includes a transformer 94 having secondary windings 85 and 96and unidirectional conducting devices 91 and 98 which rectify thealternating voltages of peaked wave form generated by transformer 94.Transformer 94 is preferably of the saturable type designed to generatevoltages of peaked wave form and may be energized from the alternatingcurrent circuit I through any conventional phase shifting arrangementsuch as the rotary phase shifter 89. Electric valve 88 controls thealternating voltage of peaked wave form and in conjunction with theunidirectional devices 91 and 98 energizes primary winding IOI oftransformer I00 in opposite directions to in- -duce a voltage of peakedwave form in secondary windings I02 and I03, which in turn energizeexcitation circuits I04 and I05 associated with electric valves I and 8,respectively. The voltages impressed on control members II of electricvalves I and 8 may be controlled by adjusting phase shifter 99 to effectcontrol of the amount of energy transmitted to the load circuit 2 duringeach half cycle of voltage applied to electric valves 1 and 8 during theperiods of conduction thereof.

Each of the excitation circuits I04 and I05 includes an electric valveI06 which is energized in accordance with the voltage appearing acrossthe anode 8 and the cathode I0 of the associated main electric valve andcomprises an anode I01, a cathode I08, a control member, such as a gridI09, and a screen grid H0. The screen grid IIO may be connected to thecathode or to any potential less positive than that of the anode I01.Electric valves I08 are preferably of the type employing an ionizablemedium, such as a gas or a vapor, and to serve as a protective means forthese electric valves we employ a suitable current limiting arrangementsuch as a fuse I I I and a serially connected resistance II2. To impresson the grids I08 of electric valves I06 negative unidirectional biasingpotentials, we employ any conventional arrangements such as rectifiers II3 each including a transformer II4, unidirectional conducting devicesII5, a capacitance II8 connected across the output of the rectifier anda voltage divider III. A predetermined component of the output voltageof the respective rectifiers I I3 is impressed on the associated gridI09 through a circuit including serially connected resistances H8 andH9. A capacitance I20 is connected in each of the excitation circuitsI04 and I05 and serves to absorb extraneous transient voltages inducedin windings I02 and I03 of transformer I00 when electric valves I06become nonconductive. The capacitance I 20 also serves to prevent thetransfer of extraneous transient voltages from excitation circuits I04and I05 to transformer I00. When electric valve I06 in either excitationcircuit I04 or I05 becomes nonconductive, there is impressed in theassociated excitation circuit through the associated grid I08 a voltageof steep wave front which, were it not for the presence of capacitanceI20, would be reflected to the transformer I and hence to the otherexcitation circuit. A capacitance I2I is connected across grid I00 andcathode I08 to absorb extraneous transient voltages. The voltages ofpeaked wave form generated by windings I02 and I03 of transformer I00are impressed across the terminals of resistances H8 in excitationcircuits I05 and I04, respectively, and are of a magnitude sufficient toovercome the negative unidirectional biasing potentials to renderelectric valves I06 conductive at the proper times.

The operation of the embodiment of our invention diagrammatically shownin Fig. 1 of the accompanying drawings will be explained when the systemis operating to effect periodic energization of the load circuit 2. Theswitch 6, of course, is moved to closed circuit position and theelectric valves I and 8 operate to control the voltage impressed onprimary winding 4 of transformer 3 to control the energization ofcircuit 2. Let it be assumed that it is desirable to energize the loadcircuit 2 periodically and that it is desirable to initiate consecutiveenergizations thereof during half cycles of voltage of oppositepolarity,

of the alternating current circuit I. To effect this result the switch43 of circuit I2 is moved to the right-hand position so that thevoltages impressed on control members 30 of electric valves 24 and 26are displaced 180 electrical degrees. The manner in which this circuitoperates to effect this type of control is explained in detail in theabove mentioned copending patent application of Harold W. Lord, SerialNo. 146,019. Furthermore, let it be assumed that the switch 53 is in theposition shown in the drawings so that the circuit I2 generates a trainof electrical impulses of predetermined periodicity. The periods ofthese impulses are determined primarily by the value of the capacitance3|, impedances 25 and 21, and the number of the impedances, includingcapacitances 36 and 31 and resistances 38 and 39, which are connected inthe circuit I2. The circuit I2 will generate the periodic electricalimpulses due to the alternate charging of capacitance 3| from the directcurrent source I3 through electric valves 24 and 26, and the anodecurrents transmitted by these electric valves will induce an alternatingvoltage in the secondary winding 34 of transformer 32. The alternatingcurrent output of secondary winding 84 is rectified by the rectifier I9and impresses unidirectional impulses of voltage on control member ofelectric valve 12 to render electric valve I2 conductive in accordancewith the periodicity established by circuit I2.

Circuit 61 operates to generate a periodic 'control voltage of apredetermined period in response to the electrical impulses generated bythe circuit I2. Capacitance 68, which is charged from the direct currentsource I3, is periodically discharged through the circuit includinginductance II and electric valve 12, and the period of the periodicvoltage generated by circuit 61 may be controlled by adjustingresistance I0. The periodic voltage generated by circuit 61 is impressedacross control member 88 and cathode 92 of electric valve 88 in circuit81, and renders electric valve 88 conductive for a predetermined time topermit the transmission of a predetermined number .of impulses ofvoltage of peaked wave form, which in turn render electric valves 1 and8 conductive for a corresponding interval of time.

The operation of the arrangement-of our invention shown in Fig. 1 may bebetter understood by considering the operating characteristicsrepresented in Fig. 2 where curve A represents the alternating voltageof supply circuit I, and curve B represents the periodic electricalquantities generated by circuit I2 which determine the periodicity ofthe energizations of load circuit I2. Curve C represents the periodiccontrol voltage generated by circuit 61 and which renders electric valve88 in circuit 81 conductive for a predetermined number of half cycles ofvoltage of circuit I. Curves D and E represent the alternating voltagesimpressed across anode 9 and cathode I0 01 electric valves I and 8,respectively, and curve F represents the alternating voltages of peakedwave form generated in secondary windings I02 and I03 of transformer I00and which are impressed on grids I08 of electric valves I06 to renderelectric valves 1 and 8 conductive. Spaces G represent the current inthe load circuit 2 due to conduction of current by electric valve I, andspaces H represent the current in the load circuit 2 due to theconduction of current by electric valve 8. Curves G and H represent thecurrent transmitted to the load circuit 2 when circuit 81 is adjusted tocontrol the amount of energy transmitted to the load circuit during thehalf cycles of energization of the load circuit 2. It is to be notedthat although the period of electrical impulses generated by circuit I2as represented by curve B may be relatively large, the periodic controlvoltage generated by circuit 61 is relatively small so that the intervalof energization of the load circuit 2 is accurately determinable, andthe ratio of the intervals of conduction to the intervals ofnonconduction is also accurately determinable and controllable. As anadditional matter, it is to be noted that the consecutive energizationsof the load circuit 2 are initiated during half cycles of voltage ofopposite or like polarity of the alternating current circuit I asdesired by the operator.

If it is desired to effect onlyone period of energization of the loadcircuit 2 in response to a single circuit controlling operation, theswitch 53 may be moved to the right-hand position to connect effectivelycircuit 54 to the system. The circuit 54 then impresses oncontrolmembers 30 of electric valves 24 and 26, through resistances 5| or 52,64, 6i and 58, a negative potential sufficient to render ineffective thealternating voltages generated by transformer 45. When the switch 60 ismoved to the open circuit position, the negative unidirectional biasingpotential impressed on control members 30 of electric valves 24 and 28is temporarily decreased to permit the alternating voltages generated bytransformer to render electric valve 24 or 26 conductive. The switch 68is normally maintained in the closed circuit position and when in thatposition the negative biasing potential supplied by circuit 54 is ofsufficient magnitude to render ineffective the voltage supplied bytransformer 45. Upon closing the switch 60 after an initiatingoperation, the circuit 54 is restored to its initial conditionpermitting a repetition of the above described sequence of operation.The operation of circuit 54 is more fully explained in the aboveidentified copending application of Harold W. Lord, Serial No. 138,809.

While we have shown and described our invention as applied to aparticular system of connections and as embodying various devicesdiagrammatically shown, it will be obvious to those skilled in the artthat changes and modifications may be made without departing from ourinvention, and

we, therefore, aim in the appended claims to cover all such changes andmodifications as fall the true spirit and scope 01' our invention.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. In combination, an alternating current supply circuit, a loadcircuit, electric valve-translating apparatus connected between saidcircuits for effecting periodic energization of said load circuit bytransmitting trains of electrical impulses thereto, means for generatinga periodic electrical quantity to control the periodicity of said trainsof impulses transmitted to said load circuit, and independent meansresponsive to said electrical quantity for generating a periodic voltageto con-, trol the ratio of the intervals of conduction to the intervalsof nonconduction of said electric valve translating apparatus.

2. In combination, an alternating current supply circuit, a loadcircuit, electric valve translat ing apparatus connected between saidcircuits for eflecting periodic energization of said load circuit bytransmitting trains of electrical impulses thereto, a circuit forgenerating a periodic electrical quantity to control the periodicity ofsaid trains of impulses transmitted to said load circuit, and a circuitcomprising a source of direct current, a capacitance and an electricvalve responsive to said electrical quantity for generating a periodicvoltage to control the ratio of the intervals of conduction to theintervals of nonconduction or said electric valve translating apparatus.

3. In combination, an alternating current supply circuit, a loadcircuit, electric valve translating apparatus interposed between saidcircuits for transmitting energy therebetween, a source of directcurrent, a circuit energized from said source for-determining theperiodicity of the energization of said load circuit by said electricvalve translating apparatus comprising a capacitance and an electricvalve means for effecting periodic charge and discharge of saidcapacitance to provide a periodic electrical quantity, and a circuitincluding a capacitance arranged to be charged from said source ofdirect current and an electric valve for eil'ecting discharge of saidsecond mentioned capacitance in accordance with said period- 10electrical quantity for controlling the conduc tivity of said electricvalve translating apparatus to control the duration of the periods ofenergization of said load circuit.

4. In combination, an alternating current supply circuit, a loadcircuit, electric translating apparatus includingelectric valve meanshaving a control member for effecting periodic energization of said loadcircuit by transmitting trainsvof electrical impulses thereto, a circuitfor generating a periodic electrical quantity to control the periodicityof said trains of impulses transmitted to said load circuit and acircuit comprising a source of direct current, a capacitance and anelectric valve responsive to said electrical quantity for impressing onsaid control member of said electric valve means a periodic controlvoltage to control the interval of time between consecutiveenergizations of said load circuit.

5. In combination, an alternating current supply circuit, a loadcircuit, electric translating apparatus comprising electric valve meanshaving a control member for eflecting periodic energization of said loadcircuit by transmitting trains of electrical impulses thereto, anexcitation circuit for energizing said control member comprising meansfor impressing thereon an alternating voltage of peaked wave-form, acircuit 101' generating 2. 6 0 within a periodic electrical quantity tocontrol the periodicity of said trains of impulses transmitted to saidload circuit, and a circuit comprising a source of direct current, acapacitance and an electric valve responsive to said electrical quantityfor generating a periodic voltage to control said excitation circuit.

6. In combination, an alternating current supply circuit, a loadcircuit, electric translating apparatus including electric valve meanshaving a control member for eifec'ting periodic energize,- tion' of saidload circuit by transmitting trains of electrical impulses thereto, anexcitation circuit for energizing said control member comprising asource of alternating voltage of peaked wave form and a single electricvalve having a control member'for controlling the number of half cyclesof voltage of peaked wave form impressed on said first mentioned controlmember, a circuit for generating a periodic electrical quantity tocontrol the periodicity of the energization of said load circuit, and 'acircuit comprising a source of direct current, a capacitance connectedto be charged from said source and an electric valve responsive to saidelectrical quantity for periodically discharging said capacitance toimpress a periodic voltage on the control member of the electric valvein said excitation circuit to control the maximum consecutive number ofhalf cycles of voltage of peaked wave form transmitted thereby.

7. In combination, a source of direct current, a circuit including apair of parallel electric paths connected to said source of directcurrent and each including a serially connected impedance element and anelectric valve, a capacitance connected across said parallel pathsarranged to be charged alternately in opposite directions through theelectric valves for generating a periodic electrical quantity, a secondcircuit for generating a periodic voltage comprising a. capacitanceconnected to be charged from said source of direct current and anelectric valve means for periodically discharging said second mentionedcapacitance in accordance with said periodic electrical quantity, andmeans for controlling said second circuit to control the period of saidperiodic voltage.

8. In combination, an alternating current supply circuit, a loadcircuit, electric valve translating appa atus for eflecting periodicenergization of said load circuit from said supply circuit, a source ofdirect current, a circuit energized from said source of direct currentcomprising a pair 01' parallel electric paths each including an electricvalve having a control member, a capacitance connected across said pathsand arranged to be charged alternately in opposite directions throughthe electric valves and means for selectively energizing said controlmembers to generate a periodic electrical quantity consecutive impulsesof which occur during half cycles of voltage of selected polarity 01said supply circuit, and a circuit including a capacitance connected tobe charged from said source and an electric valve responsive to saidperiodic electrical quantity for periodically discharging said secondmentioned capacitance to supply a periodic

